You can configure jss app to use same namespace, like templates/project/app1 etc. Instead of application name based locations. But you need to share code between this apps for templates and components (elements you choose to share) .
Check default JSS app config for options you can configure. You can share almost everything including content, config, page structure, media.
Drawback is that you share everything of given item type or nothing since this items are in that same place.
Consider if this is still 2 apps or just one with different content.